Working with an in vitro model for invasion of red cells by malaria merozoites, this study has demonstrated that human red cells lacking the Duffy a and b antigens are resistant to invasion. This corresponds well with epidemiologic studies which show that approximately 70% of West African and American blacks are resistant to Vivax malaria. A similar proportion of these blacks are Duffy a and b negative whereas the Duffy negative genotype is extremely rare in other racial groups. The close association between the Duffy negative genotype and resistance to Vivax malaria has also been confirmed by retrospective analysis of 11 volunteers experimentally inoculated with P. vivax. A retrospective study of U.S. servicemen who contracted malaria in South East Asia has been completed and confirmed again our original hypothesis. A second study conducted by CDC in Honduras has also been completed and is another in-vivo study in-proof.